I am with Xandros9 on this, but it's best with an explanation.
The Developers Preview Program was set up so that developers could test new updates before general release, when Microsoft have an update, they also have to let the carriers (Vodafone, EE etc) have a look and make sure it all works with their parts of the phone (Normally crap that no one uses) but this delays the release, if MS can already have Cortana on new phones, it should work on all of them.
So MS came up with the idea of letting anyone join the developers preview program and although you don't get the whole update that is coming, you do get stuff like Cortana, so to go into that a bit deeper, you don't get the Firmware part of an update that are called by names, so Black, Cyan and the latest that is on it's way Denim, but you do get any other parts like Cortana and nestable tiles.
From my understanding as of recently going on the DPP is not as much of a jump as it used to be, so when the Firmware comes out you will end up with exactly the same update as if you waited.
However you do have to understand when you do an update there is a warning, you do it at your own risk. However as mentioned after months of waiting for Cortana my lad bought a 930 and it had Cortana installed and I was a bit miffed, so I signed up, it took about an hour I suppose as three full updates had to download and install, on a slower phone this could take an evening, but in my opinion it was worth it as I am now out of the very slow hands of the carriers.